Field of the Invention
The present invention relates to an electrophotographic image forming apparatus that includes a fixing device.
Description of the Related Art
In an image forming apparatus (such as a printer, a copying machine, or a facsimile machine) using an electrophotographic process technology, an electrostatic latent image is normally formed on the surface of a photoreceptor (a photosensitive drum, for example) when the photoreceptor that is uniformly charged is illuminated with (exposed to) laser light based on image data. Toner is then applied onto the photoreceptor having the electrostatic latent image formed thereon, so that the electrostatic latent image is visualized to forma toner image. After transferred directly onto a paper sheet or indirectly onto a paper sheet via an intermediate transfer member, this toner image is heated and pressed by a fixing device, to form an image on the paper sheet.
The fixing device includes an upper fixing unit that has a fixing surface side member placed on the image formation side (the side on which a toner image is formed) of a paper sheet, a lower fixing unit that has a back surface side supporting member placed on the back surface side (the opposite side from the fixing surface) of the paper sheet, a heat source, and the like. As the back surface side supporting member is pressed against the fixing surface side member, a fixing nip for nipping and conveying a paper sheet is formed.
In the fixing device, the surface temperature (hereinafter referred to as the “fixing temperature”) of the fixing surface side member is controlled so as to supply a predetermined amount of heat to a paper sheet. Specifically, as shown in FIG. 1, outputs of the heat source are controlled so that the fixing temperature is maintained within a predetermined allowable fixing temperature range (from a target fixing temperature to the temperature 8° C. higher than the target fixing temperature). The target fixing temperature is an index for controlling outputs of the heat source. When the image forming apparatus is on, or when the operation mode is switched from a power saving mode, or when paper types to be used in image formation are changed, or the like, a warming-up operation for the fixing device (hereinafter referred to as the “warming-up for fixing”) is performed in accordance with the set allowable fixing temperature range if the current fixing temperature is outside the allowable fixing temperature range.
The allowable fixing temperature range is set so that neither low-temperature offsetting nor high-temperature offsetting will occur. Low-temperature offsetting is adherence of toner to the fixing surface side member due to insufficient adhesion between the toner and the paper sheet in a case where the toner does not melt well due to a shortage of heat. High-temperature offsetting is adherence of a broken toner layer to the fixing surface side member due to a decrease in cohesive force of the toner in a case where the viscosity of the toner becomes too low due to heating. As shown in FIG. 2, a fixing allowing range is from the temperature at which low-temperature offsetting occurs to the temperature at which high-temperature offsetting occurs. However, the temperature that is higher than the low-temperature offsetting temperature by a predetermined margin (10° C., for example) is normally set as the lower limit of the allowable fixing temperature range, and the temperature that is lower than the high-temperature offsetting temperature by a predetermined margin (10° C., for example) is normally set as the upper limit of the allowable fixing temperature range. The amount of heat required for fixing varies with types, basis weights, and the like of paper sheets, and therefore, the allowable fixing temperature range also varies with types, basis weights, and the like of paper sheets.
So as to increase productivity, the warming-up for fixing is preferably completed in a short period of time. In view of this, there is a suggested image forming apparatus that sets a lower or higher fixing speed (the speed of conveyance of paper sheets passing through the fixing device) than the fixing speed for regular image formation, to make the allowable fixing temperature range wider than that for regular image formation and shorten the warming-up time before fixing (JP 2004-145086 A and JP 2003-66765 A, for example). There is also a suggested image forming apparatus that makes the allowable fixing temperature range wider than that for regular image formation and shortens the warming-up time before fixing in a case where image formation is performed on a small number of paper sheets or where images that require a small amount of toner are formed (JP 2003-186346 A and JP 11-125987 A, for example).
Meanwhile, an image forming apparatus has various kinds of adjustment modes for adjusting positions, densities, and the like of images to be output. In an adjustment mode, various kinds of adjustments are performed based on an output of a pattern image for adjustment. Therefore, if the fixing temperature is outside the allowable fixing temperature range, the warming-up for fixing is performed in the same manner as the warming-up for fixing performed at a start of regular image formation. The allowable fixing temperature range for adjustment is normally the same as the allowable fixing temperature range for regular image formation.
In such an adjustment mode, the warming-up time before fixing can be shortened by using a technology disclosed in one of the above mentioned literatures, which are JP 2004-145086 A, JP 2003-66765 A, JP 2003-186346 A, and JP 11-125987 A. However, the allowable fixing temperature range is set so that image fixing properties can be secured or the fixing properties will become 100%. Therefore, the allowable fixing temperature range is not dramatically extended, and the effect to shorten the warming-up time is limited.